Fuel economy is a popular subject these days, and many advertisements talk about how their car is thrifty, miserly, stingy, (or a whole host of other synonyms for "efficient") with its fuel use.

That may well be, but as a whole, Internal Combustion Engines (ICE's) are remarkably fuel inefficient as it turns out. I kinda suspected that, but I was surpised at exactly how inefficient the ol' car engine is. Of all the chemical energy locked up in a gallon of gasoline, only about fifteen percent actually makes the car go forward.

Fifteen percent. A meager one-sixth of all the energy in the gasoline. Let's try to understand this. Here in California, gasoline has passed the four dollar mark (and I imagine the rest of the country isn't far behind.) So, given that price, let's see where your money is going. According to the CSAA, out of that $4....

About $2.48, (roughly 62%) is lost to heat. In other words, you're paying two and a half dollars per gallon to make your engine block hot.

68 cents (17%) is lost keeping your engine running while you're waiting for the light to turn green, or any other time you're standing still.

24 cents is lost through the driveline loss: that's friction in the transmission as gears transfer energy from the engine to the wheels.

Finally, only 60 cents (15%) of your four dollars is actually used to make your car get from point A to point B.

How can we make this better? Ideally, we'd need to get away from the Internal Combustion Engine. (Being a railfan, my personal druthers are to go with mass transit...) But Americans love their cars, so that's probably not going to happen any time soon. Assuming the Steel Centaur culture is a fact of life for now, what can we do to make us centaurs at least a bit more efficient?

From the practical near-term standpoint, there's probably not much that we can do about the 62% heat loss. Internal Combustion is basically a bunch of timed little explosions, and that just creates a bunch of heat that we probably can't harness right now. Bright thinkers in the future might be able to convert some of that heat to mechanical energy (for example, creating steam, and from that, generating electricity to turn a motor.) But not now.

And drivetrain loss is fairly minimal; no matter how much effort is put into making that better, you've only got one twentieth of the energy equation in play. No big returns on investment here. Just keep your tires properly inflated and top off the transmission fluid.

But let's think about idle loss. Sure, traffic jams are a part of that. Another big part is being stuck behind red lights. Let's imagine that this represents a third of the 17 percent, or about 6 percent of overall energy distribution. If we could find ways to stop less—say, by better street-light timing—and so cut the time we spend behind a light in half, that would be 3% more energy used to actually go somewhere, boosting that measly 15% figure to 18%: a 20% increase in fuel efficiency (at least for city driving.)

What I like about this discussion is that it's not just about MPG, but PMPG— Passenger Miles Per Gallon.

Here's a jewel:

Amusingly, a Toyota Prius with all seats filled is nearly as efficient as a full highway coach. This is really a testament to the engineering of the Prius. The other thing of note is that the Prius is more efficient in city driving than highway driving, no doubt due to less use of the internal combustion engine, less drag due to lower speeds, and more energy recovery through regenerative braking.